Preparation of impermanent patterns of compacted frozen mercury powder particles



June 28, 1955 5 J SINDEBAND 2,711,570

PREPARATION OF IMPERMANENT PATTERNS OF COMPACTED FROZEN MERCURY POWDERPARTICLES Filed June 4, 1952 1N VEN TOR. 6'. J. /n/oeamvo PREPARATIGN FIMPERMANENT PATTERNS OF CGMPACTED FRGZEN ll/iERCURY POWDER PARTICLESSeymour J. Sindeband, Chappaqua, N. Y., assignor to Mercast Corporation,a corporation of Delaware Application June 4, 1952, Serial No. 291,643

7 Claims. (51. 22-195) This invention relates to impermanent ordisposable patterns which are used in forming shell molds for precisioncasting, to the preparation of such patterns and to the preparation ofshell molds made with such patterns.

Impermanent patterns formed of frozen mercury, as disclosed in KohlPatent 2,400,831, have been found of great value in the precisioncasting art because they make it possible to produce thin'walled porousshell molds with cavities of fine surface finish, which in turn, make itpossible to produce out of cast metals having a high melting pointintricate shaped articles such as hollow turbine buckets, or the likewith a fine surface finish, and also other high precision cast articlessuch as wave guides which are cast out of aluminum.

Heretofore, frozen mercury patterns have been produced by pouring liquidmercury into a partible master mold, the pattern cavity of which waslubricated with a lubricant such as acetone. The liquid mercury was thenfrozen in the mold cavity whereupon the mold was opened and the frozenmercury pattern removed therefrom for use in forming with it the desiredshell mold.

A major cost item in the manufacture of precision cast articles by shellmolds formed with frozen mercury patterns is the cost of making thefrozen mercury pattern out of liquid mercury poured into a partiblemaster mold. Among the factors responsible for the high cost ofmanufacturing frozen mercury patterns is the fact that when formingcomplex thin walled frozen mercury patterns, the liquid mercury must befrozen slowly in a rather laborious way to avoid shrinkage. In addition,the liquid mercury must be frozen in expensive dies made of a materialsuch as hardened tool steel or stainless steel to avoid contamination ofthe mercury, as most other metals amalgamate with the liquid mercury.

Among the objects of the invention is a process for producing a frozenmercury pattern for precision casting which eliminates the laborious andrelatively slow process of forming such patterns involving pouring of.pure liquid mercury into a partible master mold and then slowlyfreezing it therein, and a novel frozen mercury pattern produced by suchimproved process of the invention.

The foregoing and other objects of the invention will be best understoodfrom the following description of exemplifications of the same,reference being had to the accompanying drawings wherein the drawing is2. diagrammatic side view of an apparatus illustrating one method ofpreparing powdered mercury particles for use in the process of theinvention.

Impermanent patterns formed of frozen mercury in the manner disclosed inthe Kohl Patent 2,400,831 have been found of great value in precisioncasting of complicated shaped objects, such as turbine buckets and waveguides because the very small dimensional change of frozen mercury nearits melting or freezing temperatures makes it possible to form on frozenmercury patterns thin walled molds that would crack if formed around aPatented June 28, 1955 ice pattern of wax or like material which expandsto a substantial extent when it is heated to a melting or decompositiontemperature. Such frozen mercury patterns are also of great value inprecision casting because they make it possible to form therewith thinwalled shell molds with much finer refractory particle material, such aszirconium silicate, stabilized and unstabilized zirconium oxide,beryllium oxide and the like, resulting in mold cavities of very finesurface finish yielding castings of a correspondingly fine surfacefinish, and which have also the property of exhibiting high thermalshock resistance and of not cracking when casting into them metals ofhigh melting point, such as stainless steel alloys. Another outstandingadvantage of such thin shell molds obtainable with frozen mercurypatterns is the fact that they may be produced to exhibit high porositywhich is of great advantage in providing for the escape of gasesdeveloped when molten metal of high melting point is poured into themold cavity.

However, as explained above, the cost of making patterns of pure frozenmercury for use in making precision cast metal parts was heretofore amajor item in the cost of making therewith precision cast metalarticles.

The present invention is based on the discovery that the production offrozen mercury patterns for precision casting molds of the typedescribed above may be greatly simplified and much more economical andthe process for producing the desired frozen mercury patterns greatlysimplified by providing the required mercury in the form of frozenmercury powder particles which are used for compacting them into theproperly shaped frozen mercury patterns by procedures simi ar to the hotpressing processes used for forming out of refractory metal powderparticles shaped articles of desired characteristics.

According to the invention, frozen mercury powder particles arecompacted into a solid pattern body of the desired shape by conventionalcompacting dies while the frozen mercury particles are maintained at atemperature in the range between about to C. below the freezingtemperature of the mercury. A compacting pressure of .5 to 3.5 t. s. i.(tons per square inch) is sufficient for uniting or bonding a body ofsuch frozen mercury particles into a solid frozen mercury pattern of therequired strength and finish which may be used for forming therewiththin-walled porous shell molds of the same desired characteristics asobtainable prior to the invention only with frozen mercury patternsproduced by freezing liquid mercury in the properly shaped partiblemaster mold of stainless steel material.

Since the frozen mercury powder particles are not in a liquid statewhile they are compacted into the solid pattern, the compacting die maybe made of metals which would amalgamate with liquid mercury, most suchmetals being immune to frozen mercury, and having the property of notamalgamating or reacting with frozen mercury.

As a result, the compacting dies required for forming out of the frozenmercury powder particles, the desired solid pattern may be made of anyof the known readily machinable relatively soft metals and metal alloysat a fraction of the cost required for producing out of hard stainlesssteel partible master molds heretofore required for slowly freezingtherein the liquid mercury into the required frozen mercury patterns.

In accordance with the invention, the required frozen mercury powderparticles are produced by atomizing liquid mercury which has been cooledto a temperatur somewhat higher than the freezing temperature ofmercury, for instance, 1 to 4 above the freezing temperature of mercurywhich is about 40 C. at atmospheric pres sures. So precooled liquidmercury may be atomized into line frozen mercury as by discharging itthrough a, nozzle with a stream of compressed fluid cooled down orsupercooled to a temperature materially below the freezing temperatureof the mercury, and discharging the so atomized frozen mercury powderparticles into a vessel wherein it is subjected to and maintained at atemperature considerably below the freezing temperature of the mercurypowder particles under conditions which prevent their agglomerating ormerging into a continuous mass of frozen mercury.

Inert gases such as carbon dioxide, nitrogen or helium, may be in suchsupercooled condition for atomizing such precooled liquid mercury whichis near its freezing point into powdered frozen mercury powderparticles.

In accordance with the invention, the atomized frozen mercury powderparticles are discharged into a liquid maintained at a temperatureconsiderably below the freezing temperature of mercury as a part of theprocess in which precooled liquid mercury which is on the verge offreezing is atomized into frozen mercury powder particles.

. In accordance with a phase of the invention, the required frozenmercury powder particles are produced by atomizing liquid mercury whichhas been precooled to near freezing in the manner described above with astream of supercooled liquid which is supercooled to a temperatureconsiderably below the freezing temperature of mercury. The supercooledatomizing liquid is discharged with high velocity through a nozzle towhich the precooled liquid mercury is delivered so that the highvelocity supercooled atomizing liquid causes the dispersion of theprecooled liquid mercury as it passes through the atomizing nozzle. Asin the atomizing process described above, the atomized frozen mercurypowder particles are discharged into a supercooled liquid maintained ata temperature considerably below the temperature'of frozen mercury sothat the individual particles of the frozen mercury powder areindividually enveloped and kept separated by films of the storing liquidwhich sup presses any tendency of the individual frozen mercuryparticles to merge or agglomerate.

Solid carbon dioxide sublimes into gas at -78.5 C. Accordingly, whencarbon dioxide gas is used for atomizing the precooled mercury, thecarbon dioxide gas may be cooled to a temperature much lower than 40 C.,the freezing temperature of mercury. Nitrogen or helium gas may beprecooled to very low temperatures for use as an atomizing gas, thefreezing point of the two gases being ---209 C. and 272.2 C.,respectively.

The following liquids may be precooled to temperatures considerablybelow the temperature of frozen mercury for use as an atomizing liquidor as a storing liquid for atomizing and/or storing the atomized frozenmercury powder particles.

Freezing Chemical N ame Trade Name Temperature CDiehlorotetrofluoroethane Freon 114. -95 Acetone 95Monochloropentafiuoroeth an Freon 11 -106 TrlchloromonofiuoromethaneFreon 11 11l Diehloromonofluoromethane- Freon 21 135Dichlorodifluoromethane Freon 12 1 58 Monoehl0rod1fluoromethane Freon22- -160 Monochlorotrifluoromethane. Freon 13- --181 Tetrafluoromethane.Freon 14 -184 Nitrogen -209 Helium 272. 2

There will now be described, by way of example, one form of arrangementfor atomizing precooled liquid mercury which is near freezing intofrozen mercury powder particles which are maintained in powder formready for use in practicing the invention, the drawing showing indiagrammatic form the general arrangement of an equipment suitable forthis purpose. It comprises a vessel structure generally designated 20containing an inner vessel 21 which is cooled by any conventionalrefrigerating or cooling equipment surrounding or associated with it soas to maintain its interior vessel space at a supercooled temperatureconsiderably below the normal freezing temperature of mercury. A body ofsupercooled storing liquid 22 is arranged to be retained within thevessel at a suitable level indicated at 22-1. The storing liquid 22 ismaintained at a desired sub-cool temperature considerably below thefreezing temperature of mercury either by circulating it through a suitable refrigerating system not shown or by circulating a supercooledrefrigerant fluid through pipes (not shown) immersed in the storingliquid 22.

In the inner part of the vessel 21 there is mounted a discharge duct 31terminating in a discharge nozzle 32 having a discharge opening 33 fordischarging therethrough a thin stream of precooled liquid mercurymaintained near its freezing temperature. The precooled liquid mercuryis supplied to the discharge duct 31 by a supply duct 34 suitablyenclosed by a heat insulating enclosure 34-1 to assure that mercury ofthe desired precooled temperature slightly above or near its freezingtemperature will flow through the discharge duct 31 into the dischargenozzle opening 33.

The liquid mercury discharge duct 31 is combined with an atomizing fluiddischarge duct 41 provided with a discharge nozzle opening 43 arrangedto discharge the atomizing fluid with sufliciently high velocity by wayof the mercury discharge nozzle opening 33 for causing the atomizingfluid to disperse the stream of precooled liquid mercury into atomizedmercury particles which are frozen as they are being atomized. Suchfreezing of the frozen atomized mercury particles may be secured eitherby discharging them through the nozzle opening 33 with an atomizingfluid of sufiicient volume and sufliciently low temperature to provide atemperature gradient and heat capacity differential which are sufficientfor securing by heat exchange the freezing of the precooled atomizedmercury particles as they are discharged into the 'vessel space 20-1.

Alternatively, or in combination with the foregoing procedure, thedesired freezing of the atomized mercury particles may be effected bydischarging them in atomized form into the body of the supercooledstoring liquid 22 within the vessel 21 for causing the atomized powderparticles entering the surface of the liquid body 22 to beinstantaneously frozen into discrete mercury powder particles.

As a further alternative and in combination with the foregoingprocedures, the desired freezing of the atomized mercury particles maybe effected by discharging them in atomized form onto a large guidesurface structure of solid material which is maintained at a very lowtemperature so that the atomized mercury powder particles reaching theguide structure are instantly frozen and guided thereover in frozen forminto the supercooled storing liquid. I

In accordance with a phase of the invention, an atomizing liquid foratomizing the precooled stream of mercury is utilized to form around theindividual atomized mercury particles a film or envelope of theatomizing liquid which is frozen together with the mercury particlesenveloped thereby giving the individual frozen atomized mercury powderparticles a protective frozen film enclosure of atomizing liquid whichis effective in suppressing merging or conglomeration of the atomizedfrozen mercury particles. The atomizing liquid used for this purpose maybe so chosen that when the frozen mercury powder particles are in asubsequent compacting stage compacted into the desired solid frozenmercury pattern, the compacting process is carried on at atemperature atwhich the protective film envelope of the mercury powder particlesbecomes liquefied and squeezed out or ejected from the die as the bodyof the frozen mercury particles is compacted therein into the desiredsolid frozen mercury pattern.

Acetone is a very effective liquid for securing combined atomizingaction and formation of protective films around the individual powderparticles in which case a storing liquid of lower freezing temperaturesuch as Freon 21 or Freon 22 is utilized as the storing fluid 22.Alternatively, any of the other liquids given in the table above, whichremain liquid at temperatures lower than the freezing temperature ofacetone, may be used as the storing liquid 22. However, the otherliquids listed in the table which have a freezing temperature in therange up to about 150 C., may be used as a film-forming atomizing liquidin the same manner as acetone, whereas liquids having a suitably higherfreezing temperature are utilized as the storing liquid.

As explained above, instead of discharging the atomized mercury powderparticles from the nozzle 32 directly into the supercooled storingliquid 22, the stream of atomized nearly frozen mercury powder particlesmay be discharged by the nozzle 32 onto a supercooled guide surfacestructure arranged to assure that the atomized mercury powder particlesreaching the guide structure are instantly rozen and guided thereover asa frozen mercury powder into the supercooled storing liquid.

The drawing shows how the atomizing equipment there of may be combinedwith such supercooled guide structure 35 of the invention. The guidestructure 35 is shown in the form of a generally curved guide or bafliewall having side walls 37 shaped and arranged to intercept the stream ofatomized mercury powder particles discharged from the nozzle 32 and toinstantaneously freeze all atomized mercury particles reaching the sameand to deflect and guide the atomized and frozen mercury powderparticles into the supercooled body of storing liquid 22 filling thelower region of the vessel 21.

The guide wall structure 35 is shown formed as a double walled hollowguide body of suitable metal, such as stainless steel, and is arrangedto circulate through its hollow interior space 35-1 a supercooledliquid, such as liquid nitrogen for assuring that the guide structure orbafiie wall 35 is at all times maintained at a low temperature whichcauses liquid atomized mercury particles discharged thereon by thenozzle 32 to become instantaneously frozen into frozen mercury powderparticles. The hollow space 35-1 of the guide structure 37 is suitablyconnected, as by circulating ducts 39 with a refrigerating circuit forcirculating properly supercooled liquid of the required low temperaturethrough the hollow guide structure 35.

In many cases, it is very difiicult or impossible to produce by theprocess of the invention a frozen mercury pattern of a complicatedobject, such as a bucket, with a simple compacting die because it isdifiicult or impossible to Withdraw the frozen mercury from the cavityspace of the opened die without distorting the shape of the desiredpattern.

To meet this difficulty, advantage is taken of the fact that frozenmercury parts at temperatures within the range from about 30 to 40 C.below the freezing point weld or unite readily into a single compositeunit when brought into contact with each other along mating surfaces.This rnakes it possible to produce frozen mercury patterns ofcomplicated objects by first preparing with the process of the inventionsimple frozen sectional parts of the desired complete frozen mercurypattern with individual compacting dies for such pattern sections.Thereafter the so formed frozen mercury pattern sections are broughtinto contact along their mating surfaces along which they immediatelyweld into the desired composite pattern having the shape of the desiredcomplicated article to be cast. Thus, for example, a frozen mercurypattern of a hollow gas turbine bucket may be produced out of two ormore frozen mercury pattern sections which are thereafter joined to eachother along mating surfaces into an integral complete frozen mercurypattern having the shape of the desired hollow gas turbine buckets.Several separate frozen mercury bucket pattern sections when broughttogether along their mating surfaces will instantly weld into anintegral frozen mercury pattern having the shape of the desired hollowgas turbine buckets.

A plurality of so produced separate frozen mercury patterns ofcomplicated objects prepared from frozen mercury powder in the mannerdescribed above may be readily joined into an integral multiple patterncluster which is used for forming a mold with a mold cavity of the shapeof the pattern cluster into which hot metal may be cast for producing ametal casting having the shape of the pattern cluster. Such frozenmercury multiple pattern cluster may be formed as follows. A row offrozen mercury bucket patterns is joined by contact welding to aplurality of lateral sprue branches of frozen mercury. The several spruebranches of frozen mercury carrying the frozen mercury bucket patternsunited thereto are then joined to a main vertical sprue stem of frozenmercury which is provided at its upper end with an enlarged gate sectionholding anchored therein a metallic hook, of material such as steel, foruse in handling the pattern cluster while it is used for forming aroundit the desired mold.

Such frozen mercury pattern cluster is then used for forming the mold,for instance, in the form of a thin porous shell mold.

An important feature of the present invention, is the fact that it makesit possible to use, what are known as fusible metals, which arerelatively soft, and readily machined as cast, for producing thecompacting dies with which the frozen mercury powder is compacted intothe desired casting pattern. Among such metals are, by way of example.Woods metal which melts at about 70 C., consisting 7 to 8 parts bismuth,4 lead, 2 tin and 1 to 2 parts cadmium; alloy of 50% bismuth, 25% tin,25% cadmium which melts at about 93 C.; alloy of 3 parts cadmium, 4%tin, 15% bismuth, and 8% lead which melts at about 70 C.; alloys of 8parts bismuth 5 to 30 parts lead and 3 to 24 parts tin which melt attemperatures from about C. to 170 C.; alloy of 27% lead, 13% tin, 50%bismuth and 10% cadmium which melts at 60 C.; also such alloys of lead,tin and cadmium which melt at temperatures in the range of C. to 330 C.(Throughout the specification and claims all proportions are given inweight unless specifically stated otherwise.) Dies made of such metal,which are cooled, prior to the compacting process, to a temperature ofabout 5 to 10 C. or more below the freezing temperature of the frozenmercury powder, exhibit at such low temperature the great physicalstrength which makes them suitable for compacting the frozen mercurypowder into the desired accurately shaped casting pattern.

The features and principles underlying the invention described above inconnection with specific exemplifications, will suggest to those skilledin the art many other modifications thereof. It is accordingly desiredthat the appended claims be construed broadly and that they shall not belimited to the specific details shown and described in connection withexemplifications thereof.

1 claim:

1. In the process of forming an impermanent pattern of frozen mercuryfor the preparation of a precision mold of a shaped object to be cast,the procedure of atomizing liquid mercury which is near its freezingtemperature into fine atomized mercury particles, freezing thesoatomized liquid mercury particles into a mass of frozen mercury powderparticles, and thereafter compacting a mass of said frozen mercurypowder particles in a shaped cavity of a compacting die maintained belowthe freezing temperature of mercury so as to form out of said mass offrozen mercury powder particles a shaped frozen mercury pattern of theobject to the cast.

2. In the process of forming an impermanent pattern of frozen mercuryfor the preparation of a precision mold of a shaped object to be cast,the procedure of atomizing liquid mercury which is near its freezingtemperature into fine atomized mercury particles, freezing theso-atomized liquid mercury particles into a mass of frozen mercurypowder particles, applying to the atomized mercury particles a liquidsubstance which is of a temperature below the freezing temperature ofmercury and which has the property of adhering as a coating to saidfrozen mercury powder particles and causing the individual frozenmercury powder particles to have an adhering exterior coating of saidsubstance, and thereafter compacting a mass of said frozen mercurypowder particles having a coating of said substance in a shaped cavityof a compacting die maintained at an intermediate temperature below thefreezing temperature of mercury and above the freezing temperature ofsaid substance so as to form out of said mass of coated frozen mercurypowder particles a shaped frozen mercury pattern of the object to becast while squeezing out the liquid coating substance from the compactedbody of the frozen coated mercury powder particles.

3. In the process of forming an impermanent pattern of frozen mercuryfor the preparation of a precision mold of a shaped object to be cast,the procedure of atomizing liquid mercury which is near its freezingtemperature into fine atomized mercury particles, freezing theso-atomized liquid mercury particles into a mass of frozen mercurypowder particles, applying to the atomized mercury particles a liquidsubstance selected from the group consisting of acetone,dichlorotetrafiuoroethane, monochloropentafluoromethane,trichloromonofluoromethane, dichloromonofiuoromethane,monochlorodifluoromethane, monochlorotrifluoromethane andtetrafluoromethane which is of a temperature below the freezingtemperature of mercury and which has the property of adhering as acoating to said frozen mercury powder particles and causing theindividual frozen mercury powder particles to have an adhering exteriorcoating of said substance, and thereafter compacting a mass of saidfrozen mercury powder particles having a coating of said substance in ashaped cavity of a compacting die maintained at an intermediatetemperature below the freezing temperature of mercury and above thefreezing temperature of said substance so as to form out of said mass ofcoated frozen mercury powder particles a shaped frozen mercury patternof the object to be cast while squeezing out the liquid coatingsubstance from the compacted body of the frozen coated mercury powderparticles.

4. In the process as claimed in claim 2, wherein the mass of frozenmercury powder particles with their surface coatings adhering theretoare, prior to compacting in the compacting die, deposited in a storingliquid which is maintained below the freezing temperature of mercury andfrom which storing liquid they are transferred into the cavity of thecompacting die.

5. In the process as claimed in claim 4, wherein said storing liquid ismaintained at a temperature below the freezing temperature of thecoating substance applied to the frozen mercury particles.

6. In the process as claimed in claim 3, wherein the mass of frozenmercury powder particles with their surfaces coatings adhering theretoare, prior to compacting in the compacting die, deposited in a storingliquid which is maintained below the freezing temperature of mercury andfrom which storing liquid they are transferred into the cavity of thecompacting die.

7. In the process as claimed in claim 6, wherein said storing liquid ismaintained at a temperature below the freezing temperature of thecoating substance applied to the frozen mercury particles.

I References Cited in the file of this patent UNITED STATES PATENTS1,355,984 Lewicki Oct. 19, 1920 2,062,374 Noel Dec. 1, 1936 2,271,264Kaufmann et al Jan. 27, 1942 2,384,892 Comstock Sept. 18, 1945 2,400,831Kohl May 21, 1946 2,587,614 Golwynne Mar. 4, 1952 OTHER REFERENCES Ser.No. 268,381, Kaufmann (A. P. C.), published July 13, 1943.

1. IN THE PROCESS FOR FORMING AN IMPERMANENT PATTERN OF FROZEN MERCURYFOR THE PREPARATION OF A PRECISION MOLD OF A SHAPED OBJECT TO BE CAST,THE PROCEDURE OF ATOMIZING LIQUID MERCURY WHICH IS NEAR ITS FREEZINGTEMPERATURE INTO FINE ATOMIZED MERCURY PARTICLES, FREEZING THESOATOMIZED LIQUID MERCURY PARTICLES INTO A MASS OF FROZEN MERCURY POWDERPARTICLES, AND THEREAFTER COMPACTING A MASS OF SAID FROZEN MERCURYPOWDER PARTICLES IN A SHAPED CAVITY OF A COMPACTING DIE MAINTAINED BELOWTHE FREEZING TEMPERATURE OF MERCURY SO AS TO FORM OUT OF SAID MASS OFFROZEN MERCURY POWDER PARTICLES A SHAPED FROZEN MERCURY PATTERN OF THEOBJECT TO THE CAST.